I wouldn't use anything that will tend to hold water. A "bag" of pool noodles has lots of surface area and will be hard to dry out
Put in a solid bulkhead, Nick suggests minicell foam, or plywood with a hatch for each end to have access. And to be able to check if there is any water inside.
Or go to yostwerks.com and build a set of vinyl flotation bags. Since you would have no bulkheads you would need some way of retaining the bags so they won't float out. The SOF boats on yostwerks have partial (ring type) bulkheads that the bag can be installed thru, inflated, and therefore are completely retained.

The vinyl bags are easy to make, with a little practice and are cheap.
Access hatches and bulkheads are more involved to build, but will be valuable for yourself when you prove to SWMBO that you now have the "chops" for something more adventuresome.